Various control systems for automated powder bagging machines have been developed for the purpose of improving the accuracy and speed of the filling cycle. Such systems are typically adapted for use with conventional or modified vertical-form-fill-seal (VFFS) machines, but may be adapted for other types of powder bagging machines as well.
U.S. Pat. No. 3,498,395, granted Mar. 3, 1970 to Henry discloses one such system which is based upon an apparatus and method for detecting the direction for a desired feed rate correction in a high speed weighing operation and applying a fixed correction during a feed rate correction period of each cycle. The required correction is determined from the weights of previously formed packages. Corrections are made to the feed rate to keep the weighing cycle in timed relationship with the remainder of the machine.
With such an apparatus and method, there is a certain degree of error built in, for it is that error which is detected downstream and furnishes the basis for making the corrections to the feed rate. Furthermore, the desired objective is a substantially constant (presumably fast) feed rate. The apparatus and method of the present invention provide for variable feed rates which are adjusted during each bag-filling cycle to achieve as nearly as possible a perfect net weight in each bag. There is no built-in error.
U.S. Pat. No. 3,985,266, granted Oct. 12, 1976 to Wright discloses a digital feeder control unit which computes the weight of material delivered by the feeder. A high frequency pulse train is applied to a motor speed controller connected to the feeder motor. When a first preset weight has been delivered, this high frequency pulse train is discontinued and a lower frequency pulse train is applied to the motor speed controller so that the feeder motor is slowed. When a second preset weight has been delivered, the control system causes the feeder to stop.
Such a control system, however, is limited to two feed rates--high and dribble, and if the high feed rate fails to deliver at least approximately 98% of the desired finished weight, it will be necessary to deliver a large quantity of material at dribble speed, thus greatly extending the length of the fill cycle. In any event, the length of the fill cycle is not a constant and therefore requires either a bagging machine capable of adjusting the bagging rate or a bagging machine set so that there is considerable dead time in each fill cycle to allow for those instances where a long dribble time is required. The apparatus and method of the present invention provide for variable and multiple feed rates within each fill cycle to achieve the fastest, most accurate, most consistent dispensing of a predetermined weight of particulate material within a fill cycle of predetermined and brief duration.
U.S. Pat. Nos. 3,889,848; 4,054,784; and 4,111,272, granted June 17, 1975, Oct. 18, 1977, and Sept. 5, 1978, respectively each to Ricciardi et al. disclose an automatically controlled weigh feeding system including a container prefilled with a substance, a device for discharging the substance at a controllable rate, apparatus for weighing the container and its contents, and means for controlling the discharge in response to the sensed weight of the container and its contents.
Although the inventions of these Ricciardi patents represent considerable improvement over the Henry invention discussed above, in that these inventions provide for instantaneous correction of feed rate in response to variations from a standard rate of loss in weight of the hopper, rather than delayed correction of feed rate in response to variations from a standard weight of filled bags downstream, still these inventions have as their objective a substantially constant (presumably fast) feed rate. The Ricciardi inventions are, therefore, not designed for use with a VFFS machine operating with a series of discrete fill cycles. On the other hand, the apparatus and method of the present invention are directed at such VFFS machines. For optimum speed and accuracy, multiple and preferably variable feed rates are required.